Evaporated solder bumps or C4's have long been in use for bonding integrated circuit chips to a next level of assembly, such as a ceramic package. Traditional solder bumps have involved evaporating a series of layers on each wafer: lead and tin are evaporated on a thin layer of gold on layers of copper, chrome-copper, and chrome. The lead and tin layers are evaporated through a single thin metal shadow mask that is aligned to vias that open to chip metallization. The shadow mask is clamped in close contact with the wafer. This process has produced a very high yield of wafers with terminal metals. However, as the number of solder bumps needed on each chip has been increasing with each generation of logic chips, the shadow mask technique has begun to reach its limit in reducing the size and more closely packing the solder bumps.
A new method has been developed to electroplate solder bumps on wafers, as described in commonly assigned U.S. Pat. No. 5,503,286 to Nye et al, incorporated herein by reference. This technique has the potential to substantially lower cost, improve the efficiency of use of the solder materials, shrink the size of each solder bump, move the solder bumps closer together, and increase the number of solder bumps that can be placed on each chip. However, the electroplated process has required more complex processing and a more complex stack of metals under the solder bumps.
This more complex electroplating process has expanded the need to correct process errors (as compared with the well-established evaporated process) so that any wafers that are misprocessed can be reworked. The difficulty has been in removing each layer of the stack of layers of metal without any of the etchants damaging other insulating or metal layers that are to remain permanently on the wafer. As described in commonly assigned U.S. Pat. No. 5,462,638, incorporated herein by reference, there is concern that etchants could seep into the chip through pinholes and damage the internal structure of the chip, particularly aluminum lines. Without an adequate solution to this problem, wafers that were misprocessed had to be scrapped. Thus, a better solution for rework is required to remove the terminal metal layers on a wafer without the danger of damaging permanent metal or insulating layers on the wafer, and this solution is provided by the following invention.